The manner in which a biological tissue sample is to be processed for a histological investigation is known. Firstly, trimming of the tissue sample and introduction into a cassette are accomplished. The sample is then prepared, by way of a plurality of chemical treatments, for microscopic investigation. In this context the sample is first fixed with a fixing medium, the water present in the sample is removed, and optionally further processing steps are completed. Infiltration of an infiltration medium, usually paraffin, into the sample comes at the end of this multi-step process.
The sample is then embedded, usually manually, into an embedding medium, for example paraffin. For this the sample is placed into a mold and the mold is filled with the initially liquid embedding medium. The embedding medium then hardens. The result is to produce an embedded block in which the sample is surrounded and immobilized in stationary fashion by the embedding medium.
After hardening of the embedding medium, the embedded block that contains the sample can be sectioned with the microtome into individual thin sample sections that are stained in a subsequent step and can then be investigated with a microscope. It is important for this purpose that the tissue sample have a specific orientation within the embedded block, in particular in order to ensure proper sectioning and to ensure that the tissue sample can be sectioned along the sample layers that are of interest for microscopic investigation.
In order to allow automation of the embedding process, it is necessary for the tissue sample to maintain its intended orientation relative to the molding vessel during the embedding operation. If a holding element that holds the tissue sample in its orientation is used, the problem exists that the holding element necessarily becomes embedded together with the tissue sample, and impedes subsequent generation of sample sections and/or even damages the microtome blade.
EP 2 322 938 B1 discloses an automated machine that is embodied for embedding tissue samples that must be arranged on very special carriers. The carriers are suitable for being sectioned together with the embedded sample by means of a microtome. The machine comprises several immovable holders, each holder being embodied to hold one of the special carriers during the entire embedding operation. Once a carrier equipped with a sample has been positioned in a holder, the carrier is filled with an embedding medium that is delivered from a dispenser. The carrier is then cooled by the carrier, which is also embodied as a cooling unit in order to implement an additional function.
Instead of a sectionable carrier it is also possible to use a special tissue cassette such as the one known from DE 10 2013 204 651 A1. This document describes an embedding process in which a special sectionable carrier is not necessary. In this embedding process, paraffin is poured into the tissue cassette and the tissue cassette is then held against a cooling surface so that the paraffin at the bottom of the tissue cassette begins to cool. As a result of the paraffin cooling in this region, a first thin layer of solidified paraffin adheres the tissue sample to the base. While the remaining paraffin is still molten, a retraction component pulls a first tissue engagement surface away from the tissue sample.
DE 10 2013 204 651 A1 concretely discloses a tissue cassette for holding a tissue sample, which comprises a retention component that has a first tissue engagement surface, and at least one preload element. The first tissue engagement surface is mounted on the retention component movably thanks to the preload element. A base comprises a second tissue engagement surface and is embodied so that it engages into the retention component in order to embody an interior space region, the first and the second tissue engagement surface facing toward one another. The preload element is configured so that it forces the first tissue engagement surface toward the second tissue engagement surface in order to retain the tissue sample between them in the interior space region. A retraction component is connected to the retention component and is embodied so that it retracts the first tissue engagement surface and compresses the preload element in order to form a gap between the tissue sample and either the first tissue engagement surface or the second tissue engagement surface.
The use of a tissue cassette of this kind is not unproblematic in practice, however, since a large number of interfering phenomena can occur which can in fact cause the tissue sample to become unusable. For example, it can happen that the tissue sample becomes inadvertently moved or damaged in the context of the retraction operation, or even that the tissue engagement surface cannot be retracted sufficiently or at all.